Don't delete the default branch (#11)
Not everyone uses `master` as the default branch so instead of guarding
against deleting `master`, we can instead guard
`Repository.DefaultBranch` which will handle what ever the project has
defined as their mainline.
Closes #7

ghb0t

A GitHub Bot to automatically delete your fork's branches after a pull request has been merged.

NOTE: This will never delete a branch named “master” AND will never delete a branch that is not owned by the current authenticated user. If the pull request is closed without merging, it will not delete it.